The gremlin, g, is a NetHackmonster. Getting them wet will cause them to divide into two gremlins with half the maximum HP as the original; they will actively seek moats and fountains in order to promote this process. In addition, at night they have the ability to steal intrinsics. They are vulnerable to light, and their flesh is poisonous.

Gremlins, unique among NetHack monsters, are harmed by sources of light. In particular, breaking a wand of light will do (1 + charges)d4 additional damage to gremlins caught in the blast. Flashing an expensive camera will do up to 4 damage to gremlins caught in the flash. [3]

If you find a Gremlin in a level with water or a fountain with telepathy, kill it quickly, or chances are you will end up having about 50 of them running around. This is especially bad for chaotics: the herd of gremlins will sometimes be generated peaceful, thus killing them inflicts a large hit on your alignment record.

On the other hand, if you find yourself with a tame gremlin but don't particularly want one, and have a polymorph trap handy, consider allowing it to split a few times so you can get more useful pets by polymorphing them. However, the polymorphed pets will be low on HP, down to 1/1 HP in the case that the gremlin was completely split. So for this strategy, the gremlin should not be split too often (apply a magic missile at a dry spot if it is enough) and the new pets should be kept away from strong monsters until their max HP is high enough.

The gremlin is a highly intelligent and completely evil
creature. It lives to torment other creatures and will go
to great lengths to inflict pain or cause injury.

Suddenly, Wilson thought about war, about the newspaper
stories which recounted the alleged existence of creatures in
the sky who plagued the Allied pilots in their duties. They
called them gremlins, he remembered. Were there, actually,
such beings? Did they, truly, exist up here, never falling,
riding on the wind, apparently of bulk and weight, yet
impervious to gravity?
He was thinking that when the man appeared again.